In a pixel-based scanner, printer, copier, or FAX machine, a swath consists of a number of pixel rows, modernly on the order of twenty to two hundred. In a swath scanning system the pixel columns within each swath are serially acquired, one pixel column at a time, by an imager and detector array which move together across the document on a carriage.
We call this type of operation "carriage scanning", "swath scanning", or "swath-type scanning"--and the devices "carriage scanners", "swath scanners", or "swath-type scanners". They are also sometimes called "moving scanners".
In this document we use the phrase "full transverse row scanner", or simply "row scanner", to mean a scanner of the earlier type that acquires pixel rows one at a time. We use the phrase "pixel line" as a general term encompassing both (1) a pixel column, most typically of length equal to the swath height, in a swath scanner; and (2) a pixel row, most typically of length equal to the image width, in a row scanner. In short, as permitted by the context a pixel line may be either a pixel column or a pixel row.
(a) Row scanners generally--A scanner of this sort is generally one of three types. One is a paper-transport type, which translates the image medium past a stationary wide, shallow window that covers the receiving optics.
A second is a flatbed type which conversely moves all or part of the optical train under or over the stationary image medium--which is often positioned face-down on, and viewed through, a transparent pane. A third type takes the form of a small handheld appliance, which contains all or most of the apparatus and is moved manually over the image medium.
Most commonly all such row scanners have utilized broadband illumination--i. e., light that is nominally white--and filters or a prism to separate the resulting white-light image from the illuminated image-bearing medium into three separated-color images for either time-shared or spatially-separated detection.
A row scanner illuminates an entire row on the original object or document--entailing relatively high power and, depending on the type of light source in use, high voltages and relatively large amounts of heat. To avoid intensity fluctuation due to voltage ripple in fluorescent sources, excitation is sometimes at high frequencies--in return requiring care to avoid electromagnetic interference with high-frequency, high-impedance signals in the scanner circuits.
In such systems normally the illumination is continuous during scanning. As sensors many of these systems use charge-coupled devices (CCDs), or integrating photodiode arrays--which are periodically read out to data-transfer circuits.
CCDs, and also some integrating photodiode arrays, are sensitive to accumulated light arriving between the periodic readouts. In such systems, with continuous illumination, special provision must be made to manage this sensitivity of the detectors to the accumulation of optical energy between readouts.
(b) Color scanner with source-based color separation and sequenced sources, in a printer--U.S. Pat. No. 4,755,877 of Vollert, assigned to Siemens AG, and also the above mentioned Pat. No. 5,532,825 of Lim and Cloutier, describe use of different-color sources for color separation, and sequencing of the sources between advances from line to line. In this way they capture the full image information with color data interleaved or time-shared line by line.
Vollert in particular relies upon light-emitting diodes (LEDs) as sources. Color LEDs are available for red, green and blue--nominally the same colors as used to define standard industrial color, but not actually emitting the same spectral distributions as standard sources. In general some calculation is required to determine color constituents that would be found if the sources were actually standard.
Technically speaking it is impossible to make such a determination exactly, since sources with different spectral distribution simply do not provide all the necessary information for a full calculation. LEDs emit in relatively narrow spectral bands that peak close to the standard wavebands but can only provide a rough approximation to the standard illumination.
Also, nothing known in the desktop scanner/printer art addresses the related fundamental problem of metameric colors (see, e. g., Judd and Wyszecki, Color in Business, Science, and Industry, Wiley 1952, 1963, at 106 ff.). In particular it is not known in the desktop scanner/printer art how to acquire color data in such a way that a scanned and then directly printed image will have a desired color appearance under incandescent lighting, for example--or under fluorescent lamps having a particular specified phosphor mix--rather than sunlight. Again of course arithmetic adjustments can be applied, but such calculations represent an additional time-consuming step.
Lim and Cloutier introduce a new type of piggyback color row scanner. Three light sources emitting three different colors are appended with receiving optics to the output paper tray of an inkjet printer; thus this is a variant of the paper-transport scanner type mentioned above.
A subsystem is provided for selecting between scanning and printing functions. Since a single image-medium transport system is used for both functions, single-pass copying is at least awkward.
Lim and Cloutier's color row scanner interleaves color data row by row. Source types mentioned include fluorescent lamps and light-emitting diodes (LEDs) For color scanning, instead an entire image for each color could be acquired separately, in sequence. This, however, would require three passes of the image-bearing medium through the apparatus and also would introduce local misregistration arising from the flexibility of most image media.
Vollert, and Lim and Cloutier, introduce a new economy into operation of a color scanner in conjunction with inkjet printing, and teach advantageous use of source-based color separation and sequenced-source color interleaving. They do leave room for refinement as to the timing demands of CCD readout, or the detailed spectra of LED colors--or more generally the metamer problem.
(c) Swath scanners--The scanner described by Vollert is a swath scanner, not a row scanner, and it is integrated into a swath inkjet printer. At the present writing, notwithstanding the early date of the Vollert patent, scanners of this type are relatively new in the marketplace.
Other swath scanners include the Cannon model CJ-10 flatbed color copier, in which an incandescent source is carried with a sensor on a transversely-translating carriage. The source is turned on substantially continuously during operation.
In the Cannon copier the sensor is believed to be a detector-column array, scanned in two dimensions. It is not integrated with the printing mechanisms--i. e., a separate printing subsystem is used for recreating on a fresh image-receiving medium the image thus acquired.
Another swath scanner-printer that does not use inkjet technology is believed to be available commercially in Japan under the Alps brand name as "Micro Dry process" (i. e., thermal-transfer) model MD-4000J. It forms images by marking with a hot wax ribbon, and may correspond to U.S. Pat. No. 4,788,587 of Bitoh, assigned to Casio.
This device carries an incandescent bulb and a sensor on the printhead carriage. The lamp evidently is turned on continuously during operation.
The Bitoh patent describes a plug-in swath scanner that is temporarily substituted for a printhead. The device carries a cyclically indexed filter belt to interleave colors swathwise, each 16-pixel swath being 2.8 mm (1/9 inch) tall.
For color scanning the device must traverse the width of the image medium to read one color of a swath, then index the filters and recross the medium to read the next color for the same swath, and finally index again and traverse the width of the image medium yet a third time to complete the 2.8 mm swath. Thus to complete a standard page the carriage beats back and forth diligently--a total of nearly three hundred passes at the rated swath height.
Even at more modern swath dimensions the total sweep count for such a system would amount some thirty to sixty-five passes. (This accounting makes no allowance for extra passes in mitigation of mechanical inaccuracies in swath abutment--discussed shortly.) Mechanical wear in such a system is a significant consideration.
Bitoh's system requires removal and replacement of pen and sensor to print or scan--and of course to copy. His system is therefore also time-consuming and inconvenient to use, just as is the plug-in system of Carbone, U.S. Pat. No. 4,525,748.
Minor awkwardness is inherent in having to set aside a pen in an office or home desk area when the scanner is installed. Maintaining mutual cleanliness can be clumsy: workspace surfaces may be soiled by ink on the printhead, and printhead nozzles may be contaminated by various debris in the workspace.
Bitoh also mentions another effort to provide some sort of convertible scan-sensor/printhead capability in a single machine. In this regard he cites Japanese Patent Disclosure 59-32833 and Japanese Utility Model Disclosure 54-133733 for a system in which "the thermal head and the image sensor are arranged so as to be offset from a sheet, that is, they are not aligned linearly with respect to the sheet."
It can be inferred that the printhead and sensor are mounted on a carriage together but with only one at a time in operating position--i. e., the printhead/sensor must be flipped in some way as part of selection between printing and sensing. Even so, Bitoh reports that "the overall reading apparatus becomes bulky."
Thus a user of these Japanese systems may face an unsatisfactory choice between Bitoh's own inherently slow plug-in system and the undesirably bulky flip-type systems he mentions.
(d) Swath scanner-printer with read/print configuration at least partially in common, and electronics integration--The Vollert patent, and the above-mentioned application of Dobbs et al., introduce swath-type scanner-printers. The Dobbs system is a plug-in black-and-white swath scanner-printer, in which plural pens and a sensor can be carried together in their respective operating positions and orientations. It also employs a degree of integration between its sensing and printing electronics.
Dobbs et al. describe an elementary form of scanner/printer integration. In scanner operation the controlling computer, or in some operating modes the printer microprocessor itself, sends a signal to (it thinks) the nozzle-firing system of the printer.
More specifically, in scanner operation the printer is instructed to fire a particular pen nozzle. The scanner module, however, receives/intercepts and uses that print instruction to instead initiate a readout sequence for the column sensor, which is a charge-coupled device (CCD) array.
In response to this print instruction, the microprocessor collects from the CCD array the data which the array has been accumulating. Those data are, namely, the optical-brightness information for a particular pixel column--the column where the CCD array has been positioned, on-the-fly within the current swath.
Part of this data-readout process, it should be noted, is reading out a series of so-called "dummy pixels". In at least some CCD hardware the dummy pixels are present and must be read out. In some systems using CCDs, some or all dummy-pixel readout is part of a regime of CCD sequencing that is essential to accommodate the previously mentioned sensitivity of the CCDs to accumulated illumination arriving between readout cycles.
Such sensitivity as noted earlier is a characteristic of some integrating photodiode arrays, as well as CCDs. In the Dobbs and Bitoh swath scanners, and prior row scanners too, as will be recalled the light sources are turned on substantially continuously during operation.
Another even more limiting accommodation of the same interreadout cumulation--considered together with the use of light sources that are on continuously--is the need to maintain constant scanning speed. Effective sensitivity of the array varies in a complicated way with duration of the interreadout interval. Commonly the least onerous way of accounting for sensitivity variation is to stabilize this interval by controlling scan speed.
Dobbs et al. mount their sensor in place of a single pen, an arrangement which--for a multipen printer, as they point out--leaves other pens undisturbed in their carriage. Thus Dobbs teaches mounting a sensor and pens together in their respective operating positions and orientations.
Inherently one of those undisturbed pens could be a pen that prints black. In that event, black-and-white scanning and printing can proceed without unplugging anything.
This is not the case for color scanning and printing. Thus the Dobbs system represents a clear advance over Bitoh and the other Japanese disclosures, for black-and-white scanning, printing, and in particular copying--but for color work there remains the above-discussed residual awkwardness of removal and plug-in.
Vollert's system is free of these drawbacks, as it includes a sensor on the carriage in addition to the full complement of four pens. Further, Vollert reduces the number of sweeps by interleaving colors, column by column, within each swath.
Nevertheless Vollert fails to teach any way of avoiding the CCD sequencing limitations described above. Moreover, neither Vollert nor Lim and Cloutier address the spectral distribution of LED sources.
For finest match to colors determined under standard illumination, LED-acquired data require application of some arithmetic conversion, in a separate processing step. As to printing of images with a desired color appearance under various specified forms of artificial light, as well, the art fails to teach methods for avoiding additional arithmetic adjustments.
Another area that is open to refinement for both the Dobbs and Vollert systems in common, notwithstanding their superiority over prior systems, is inability to copy in a single operation. On one hand, because these systems have only a single image-medium transport system and just one carriage--both of which do double duty for both scanning and printing--these two phases of copying are very economical.
On the other hand, for the selfsame reasons they cannot be simultaneous. In other words, a piece of image medium must be fed through the machine in a first operation of the medium transport to provide an image; and then a piece of image medium must be fed through the machine in a second operation of the transport to receive the image.
(e) Single-pass swath copying--Thus for copying a document or other object, Vollert, Dobbs, Lim and Bitoh rely upon a user to provide two image-medium passes through an image-related device. Of course separate scanner and printer can be provided in a single machine, to eliminate such double-pass operation, but this response nearly doubles the cost of the main working parts and so obviates the salient benefit of sharing modules in a swath scanner/printer.
Some prior artisans have attempted to mitigate this limitation by juxtaposing twin image-medium transports to a common scan mechanism. Thus U.S. Pat. No. 4,636,871 of Takato Oi proposes intermeshed transport mechanisms for the image-bearing medium and printing medium; Oi shows multiple generic "reading/printing elements" ganged to scan together across, for each element, just a portion of the image width.
The Oi system uses multiple units of particularly costly modules--namely, sensors and printheads--but putting this drawback aside, the system also requires two transport mechanisms. This requirement is adverse in terms of cost but more importantly is very demanding of tracking accuracy as between the two mechanisms.
If paper should be only slightly misfed in either half of the medium-advance system, the resulting reproduction will have a gap, or an overprinted folded-image appearance. If the mismatch is laterally asymmetrical--a twisting of either sheet in the advance system--the result may be some combination of tapered gap and overprint.
Of course paper in any system can slip or misfeed very slightly, and in fact this does occur from time to time in every kind of printer and scanner. We are accustomed to accepting such phenomena as an unavoidable occasional incident.
The Oi system, however, essentially doubles the statistical incidence of such occurrences. This development is particularly undesirable since it doubles the likelihood that a valuable original may have to be fed through the transport mechanism more than once.
Risk of damage to a valuable original document is of course an extremely important consideration in any system which moves original documents along a paper-transport path. The Oi system thus exacerbates the handicap which a moving-original copier has inherently, relative to a stationary-flatbed copier.
That handicap includes, among other things, inability to scan or copy magazines, pamphlets, and books--even those which lie relatively flat. Pages from such materials of course can be cut out and passed through a machine, but in many or most cases this amounts to an undesirable defacing or destruction of the full original book etc.
In some ways this handicap is even greater for copying of small or irregular pieces of image medium. Such originals include for example product labels and book jackets, compact-disc faces, envelopes, pieces of shipping packages, credit cards, business cards and Rolodex.RTM. or index cards, cash-register tapes, checks, and notepad sheets--and myriad other items not readily passed through most document-transport-type machines.
U.S. Pat. No. 5,162,916 of Stemmle et al. discloses a very compact, portable scanner/printer/copier--which employs an arrangement similar to that of Oi's system, as to advance of the two image media. Stemmle's two sheets are driven lengthwise above and below common drive rollers, with idlers above and below for maintaining engagement to the common drive rollers.
The same misfeed or slippage problem mentioned above as to Oi's patent remains in any such dual system. Stemmle's, however, is particularly objectionable in that his two image-medium sheets must travel in opposite directions at the same time.
The distinctly unbusinesslike appearance of such operation may be excused in a portable apparatus, and the requirement that Stemmle's apparatus must invert (with respect to the medium-advance direction) each pixel column in preparation for printing is not onerous. Nevertheless several practical drawbacks arise.
First, the two sheets of image media extend in opposite directions from the apparatus, with the original image being read facing down and the image being printed facing up. Thus the trailing edge of the original tends to drag over the leading edge of the new copy, running the risk of not only smearing the copy but also soiling the original.
This result may be disastrous if the original is a valuable document. If this system were applied in a color copying context, where relatively large quantities of liquid inks are deposited in a short time, the likelihood of damaging both documents would be particularly high, but even in black-and-white machines it is a serious concern.
Furthermore the tabletop space required by the apparatus and sheets of image media in use is somewhat longer than twice the length of a sheet of image medium. Most swath-printing machines that employ printing-medium-transport systems are only about one and a half times the length of a sheet, by virtue of curved paths for the printing medium, and this should be considered at least a goal for swath-based copiers too.
In addition the task of the user in feeding two sheets of material into the apparatus, straight, from two different directions is quite awkward. This may be mitigated somewhat by making access to the machine available at both sides of the supporting table etc.
These constraints may be problematic even in some portable-device environments. The configuration is inapposite to office or home contexts, where the economy of the basic machine is offset by floor-space and office-layout demands.
Stemmle attempts to ameliorate these limitations in his U.S. Pat. No. 5,032,922. Here the same basic device, rather than simply resting on a tabletop, is ingeniously mounted slothlike to cling to and crawl along the underside of a transparent rectangular platen. The platen is raised, by legs at its four corners, above the table.
The platen beneficially separates the original from the newly inked copy, and also enables copying of books, odd-sized documents and other objects. Potential for relative misfeed, however, remains as the copy paper is fed through the unit by a partially separate mechanism.
This potential problem is aggravated by retention of the oppositely directed paths of the platen and the printing medium, with respect to the printing apparatus itself. In this context, since the platen is stationary, the copy is ejected ahead of the traveling copier module, but traveling at double speed in the same direction as the absolute motion of the copier module relative to the table.
Due to these unfavorable relationships, it appears that the forward edge of the emerging sheet is vulnerable to being slowed or stopped by any roughness or obstacle on the table or other support. If such obstruction is only slight, it can slightly curl the printing medium in the print zone, within the copier, leading to erratic distortions of the image being formed.
If obstruction is more severe (as for instance on a cloth-covered table, or a table with other articles under the platen), the leading edge of the medium could curl back under the copier module. In this case it would be dragged inked-face-down along the support surface.
Yet another related Stemmle design appears in U.S. Pat. No. 4,920,421. The configurations discussed above may have evolved from that discussed in the '421 patent, which is a desktop model with a stationary transparent platen on a case.
Under the platen the document sensor and printhead ride together in a transversely scanning carriage, which in turn is on a gantry that also steps longitudinally. As in the '922 patent, the printing medium is ejected at double speed forward--but now safely within the case, and deposited into an output tray just at one end of the case.
This configuration too has dual motion in the advance axis: the gantry steps along under the stationary platen, and the printing medium moves through a transport mechanism on the gantry. In other words--within the common, moving inertial frame of the gantry, printhead and scan sensor--the input-document platen translates in one direction and the print medium in an opposite direction.
(f) Auxiliary print functions--The Cobbs and Sievert patent documents mentioned earlier teach use of an optical sensor and associated light source mounted with inkjet pens on a transversely operated carriage. The sensor is for use in calibrating the pens with regard to their mutual alignments in both the transverse ("scan") axis and longitudinal ("medium advance") axis.
A sensor is advantageously used on an inkjet carriage for other auxiliary functions too. These include determining whether ink supply is in need of replenishment, ascertaining the quality of printing, and locating the edge of a piece of printing medium.
As will be seen, all these functions are compatible with the teachings of the present invention. At the same time, however, it is important to clearly distinguish all such sensor uses from the present invention.
For many of these auxiliary functions, optical or spectral constraints--or both--imposed on the sensor and light sources are far less demanding than in a scanner. In most cases these modules operate on a monochrome basis, and with substantially d. c. illumination.
Swath scanners heretofore have not been associated with auxiliary print functions.
(g) The technology of print modes--In swath printing the phrase "print mode" usually refers to a particular pattern of pixels into which inking is divided, as between two or more traverses of the carriage. Print modes and so-called "masks" used in implementing them have been very highly elaborated, to serve a variety of causes.
Some print modes help to control development of excess undried liquid ink, highly localized on a printing medium at any given time. This is particularly useful with media that can absorb relatively little liquid.
Other print modes help to camouflage adverse visual effects of heating (also used to accelerate drying), or the effects of operating very near an end of the printing medium--where the advance mechanism cannot control position as well as elsewhere. Still other print modes are aimed at concealing the edges of swaths--or color shifts between swaths laid down during scanning in opposite directions--to avoid a horizontal banded appearance
To avoid such horizontal banding, and sometimes to minimize generation of moire patterns, a print mode may be constructed so that the paper advances by a fraction of a swath height between each initial-swath scan of the pen and a corresponding fill-swath scan or scans. In fact this can be done in such a way that each pen scan functions in part as an initial-swath scan (for one portion of the printing medium) and in part as a fill-swath scan.
Such techniques tend to distribute--rather than accumulate--print-mechanism error that is impossible or expensive to reduce. The result is to minimize the conspicuousness of, or in simpler terms to hide, the error at minimal cost.
Previously mentioned patent documents of Nicoloff, Hickman and particularly Cleveland survey in greater detail many uses of print modes. The Cleveland document in particular explains concepts such as "space rotation" and "sweep rotation".
These are different ways of changing the print-mode pattern or mask by shifting individual nozzle commands in each pass--or by using different patterns in different portions of the nozzle array. In addition, as Cleveland also explains, operating parameters can be selected in such a way that, in effect, rotation occurs even though the pen pattern is consistent over the whole pen array and is never changed between passes. Figuratively speaking this can be regarded as "automatic" rotation or simply "autorotation".
Heretofore the use of print modes has not been associated with scanners.
(h) Media and form identification--In both printing and scanning work for various reasons it is desirable to be able to verify automatically the character of the image medium. In printing, for example, the medium itself determines ideal inking patterns for best color appearance (saturation, color balance etc.), and also for application of related control regimens such as heating to hasten ink drying, or special end-of-page print modes to accommodate distortion that in turn results from heating.
In scanning, if the medium includes a preprinted form into which data have been typewritten, confirmation that a form is present--and identification of the specific form and its alignment--determine how the data should be interpreted, stored or reprinted. Similarly if data are to be used to fill in a new form, confirmation of the identity and alignment of the form determines how the data should be formatted for entry.
Although such automated identifications of media and forms are of considerable interest, little work along these lines has appeared in the swath-printer marketplace.
(i) Mirror images--For artistic effects and possibly for other reasons it is sometimes desirable to be able to make a mirror image of a given input object. Many graphics programs for use in computers provide for such reversals. Such mirror imaging is easily implemented electronically or in firmware.
Stemmle, for example, as mentioned above must invert each pixel column before reassembling adjacent swaths, to accommodate the oppositely directed movement of his original and copy. His apparatus performs the inversion by reading back pixels within each column in a sequence opposite to that used in the scanning stage. Such inversion is readily accomplished either in terms of firmware or in terms of direct control of a shift register.
Stemmle neglects to mention that if he did not perform this inversion each output swath, considered individually, would be a mirror image of the corresponding input swath. Since Stemmle's image-medium sheets are traveling in opposite directions, however, the overall image would not be as seen in a mirror but rather would be scrambled.
We are not aware of any scanner/printer/copier offering provision of mirror images, without intervention by an associated computer. Where mirror imaging is desired such a feature would be helpful, to save time and resources.
(j) Conclusion--Many above-discussed limitations of the related art have continued to impede achievement of uniformly excellent scanning and/or printing. These include unavailability of single-pass copying in practical form, for the most highly economical systems--and the failure to take advantage of pixel-masking techniques, integrate auxiliary functions or make mirror images.
Even the Vollert, Lim and Dobbs systems continue to contend with the fussy timing (and "dummy pixel") demands of CCD readouts, and for finest results a need for arithmetic adjustments to bring color signals into conformance with industry standards. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.